1. Field of the Invention
This invention relates to an electrochromic composition and an electrochromic display device using the same, and more particularly to an electrochromic composition containing a compound selected from the group consisting of ammonia, organic amine and alkali, and a diffusion type electrochromic display device. The "diffusion type" electrochromic display device is disclosed in United States patent application Ser. No. 769,564 and will be defined hereinafter.
2. Description of the Prior Art
Electrochromic compounds (hereinafter referred to as "EC compound") present different colors depending upon the state of the compounds, i.e. oxidized state or reduced state since the spectroscopic absorption of the compounds differs depending upon the state thereof. The EC compounds are divided into those coloring in a reduced state and those coloring in an oxidized state. The EC compounds coloring in a reduced state does not show a color or shows a light color in an oxidized state and colors in a reduced state. The EC compounds coloring in an oxidized state does not show a color or shows a light color in a reduced state and colors in an oxidized state.
The EC compounds sealed between a pair of electrodes at least one of which is transparent present color changes when application of an electric field thereacross is turned on and off. When the solubility of the EC compound dissolved in a solvent of the EC composition sealed between the pair of electrodes is low, the displayed pattern is maintained for a long time, for instance, several months. This kind of EC display device is called memory-type EC display device. When the solubility of the EC compound to the solvent is high, the displayed pattern quickly disappears upon turning off the application of the electric field. This kind of EC display device is disclosed in detail in a copending U.S. patent application Ser. No. 769,564. This kind of display device is called in said patent application "diffusion type" EC display device. The diffusion type EC display device is characterized in that between a pair of electrodes there is sealed an EC composition coloring in a reduced state which is composed of an EC compound and a solvent, the solubility of the EC compound to the solvent being not less than 10.sup.-2 mol/100 g-solvent in a reduced state and not less than 10.sup.-1 mol/100 g-solvent in an oxidized state. The EC compound is sealed etween the electrodes in a reduced state.
The diffusion type EC display device is advantageous over the known memory type EC display device (as disclosed in U.S. Pat. No. 3,854,794) in that the life time is longer, the response is higher, the driving voltage can be lower, an erasing bias voltage need not be applied, and the reference electrode need not be used.
In the diffusion type EC display device as described briefly above, it is desired that the EC compound having a far clearer color and stable color presentation be provided. It is further desired that the EC display device have a far longer life and higher contrast of the pattern displayed thereby.
In order to prepare a diffusion type EC display device, the EC compound coloring in a reduced state must be sealed in a space between a pair of electrodes in a reduced colored state. The method for sealing the EC compound in the space in a reduced state is disclosed in said patent application. For instance, the EC compound coloring in a reduced state and a solvent are sealed in the space in a reducing atmosphere as of hydrogen gas, or the EC compound coloring in a reduced state and a solvent are sealed in the space in an atmosphere of inert gas as of nitrogen gas and a high potential (coloring potential) is applied to the electrodes to convert the EC compound sealed in the space into the reducing state.
The above described methods for sealing the EC compound in the space between a pair of electrodes constituting the EC display device suffer from the defects as follows. In the former method utilizing hydrogen gas, the operation is very troublesome since all the works should be conducted in the hydrogen gas and there is a danger of combustion of the gas. In the latter method utilizing nitrogen gas, the water in the solvent is electrolyzed by the high potential and bubbles of gas generated by the electrolysis appear in the display. In order to prevent the electrolysis, the potential must be lowered and applied for a long time which results in insufficient coloring.